Raw gases such as, for example, natural gas or synthetic gas also contain, in addition to useful components, contamination such as, for example, hydrogen sulfide, carbon dioxide or organic sulfur components. Organic sulfur contaminations are primarily mercaptan and carbon oxysulfide. For use, these contaminations must be removed from the raw gases. For example, sulfur in the form of hydrogen sulfide or carbon oxysulfide is a poison for many catalysts, blocking their effectiveness. Statutory provisions also mandate lowering sulfur emission. Because of global warming, a reduction of carbon dioxide emission is also required, as its presence in natural gas also lowers heating value. For purifying raw gas of the components cited above that are described in the following as acid gases, a number of technical methods are available in which the gas is purified with an absorption agent. For removing acid gases, either chemical absorption agents or physical absorption agents can be used.
The requirements regarding degree of purity depend on the further use of the top gas and on the type of acid gas. In the case of sulfur components it is usually necessary that they be removed from the raw gas for its further technical use down to a contents in ppm. In the case of carbon dioxide, depending on the further use of the top gas, the carbon dioxide is either removed entirely, or partially, or also only as little as possible.
When removing sulfur components, the acid gas separated in the absorption agent regeneration is customarily processed further in a Claus plant into sulfur. This entails additional investment costs for the Claus plant. Because of the worldwide oversupply, no appreciable revenue can be obtained for the sulfur that is being created, so that it is difficult to amortize these investments. As an alternative to obtaining elemental sulfur, storing acid gases in gas caverns is being considered more frequently. This way, the acid gases are compressed by expensive compressors so very much that it is possible to transport the acid gases to the underground gas storage site that is provided for it, for example, an exploited natural gas field. It is therefore particularly advantageous for this purpose when the acid gases that accumulate during regeneration are present at a pressure that is as high as possible, because thereby, significant investment and operating expenses for acid-gas compression are saved. It is also advantageous for sequestering carbon dioxide if the removed carbon dioxide is recovered at as high a regeneration pressure as possible.
In the case of chemical absorption agents, the regeneration pressure at which the acid gases accumulate can only be raised marginally, as otherwise an accelerated decomposition of the chemical absorption agent would occur, because an increase in the regeneration pressure causes an increase in the boiling temperature.
In principle, in the case of physical absorption agents it is possible to obtain some of the acid gases at a higher pressure. For this, the regeneration is performed by lowering the pressure with the aid of several sequentially switched flash levels. The acid gases released at the flash levels are thereby brought to a compression level that corresponds to the corresponding pressure level of the respective flash level. Although the consumption of energy for the recompression of the acid gases can be lowered by using this approach, most of the acid gas must still be compressed from an atmospheric pressure level to the end storage pressure. Further, regeneration by lowering the pressure using flash levels makes only a limited removal of the acid-gas components possible, as a certain residual level of acid gases always remains in the absorption agent when using flash generation.
This means that the required purity of the top gas is not attained. For example, removing hydrogen sulfide according to specifications requires a purity of the top gas to a level of just a few ppms of acid-gas components. Physical absorption agents also have the disadvantage that they do not work as selectively with respect to the resource components as chemical absorption agents. In addition to the acid gases, significant amounts of usable gases are also absorbed. These usable gases are, for example, hydrogen and carbon monoxide in the production of synthetic gas, or methane in the purification of natural gas.